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Height does not impair the hydraulic system of the tallest tropical Dipterocarp trees.

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Title: Height does not impair the hydraulic system of the tallest tropical Dipterocarp trees.
Authors: Bittencourt, Paulo (AUTHOR), Scheire, Arne (AUTHOR), Jotan, Palasiah (AUTHOR), Lourenço Jr., Jehova (AUTHOR), Banin, Lindsay F. (AUTHOR), bin Suis, Mohd Aminur Faiz (AUTHOR), Burslem, David F. R. P. (AUTHOR), Christoffersen, Bradley (AUTHOR), Coomes, David (AUTHOR), Groenendijk, Peter (AUTHOR), Jansen, Steven (AUTHOR), Jucker, Tommaso (AUTHOR), Matula, Radim (AUTHOR), Mencuccini, Maurizio (AUTHOR), Oliveira, Rafael (AUTHOR), Plichta, Roman (AUTHOR), Nilus, Reuben (AUTHOR), Robert, Rolando (AUTHOR), Svátek, Martin (AUTHOR), Rowland, Lucy (AUTHOR)
Source: Science. 7/2/2026, Vol. 393 Issue 6806, p60-64. 5p.
Subjects: Hydraulics, Dipterocarpaceae, Tropical forests, Sap (Plant), Tree height, Droughts, Plant ecophysiology
Geographic Terms: Southeast Asia, Borneo
Abstract: Half of the aboveground biomass in forests is stored in a disproportionately small number of very tall trees. These giants are predicted to be more vulnerable to drought-induced damage because height impairs their hydraulic system. We evaluated whether the hydraulic system of world's tallest tropical tree species—Southeast Asian dipterocarps—are negatively affected by their height. The more negative xylem pressures caused by tree height were fully compensated for through adjustment of vessel anatomy and leaf hydraulic traits, and the trees suffered no height-related loss in growth during a severe drought. Therefore, height does not make the hydraulic systems of the world's tallest tropical tree species more vulnerable to drought, and the growth rates of these trees are not more negatively affected by drought than are their smaller counterparts. Editor's summary: Wood contains vessels that carry water from roots to distant leaves, acting like long straws. Vessels can collapse under low water pressure, which is harder to maintain at greater heights. Thus, water transport is thought to limit the maximum height of trees, and taller trees may be more vulnerable to drought. Whether these principles from temperate ecosystems extend to tropical trees, however, remains unknown. Bittencourt et al. investigated whether very tall tropical trees have traits that compensate for water stress by comparing trait measurements before, during, and after the El Niño drought period of 2023–2024 in five species of dipterocarps, the tallest known tropical tree family, in Borneo. The researchers found that hydraulic traits compensated for height-related declines in water potential, suggesting drought resilience of these massive trees. —Bianca Lopez INTRODUCTION: More than half of the carbon in forest ecosystems is stored in only 1% of the tallest trees. These tall trees are predicted to be more likely to suffer drought-induced damage under future drier climates. As a tree becomes taller, the distance from root to leaf increases, increasing the resistance to water flow. In addition, the greater effects of gravity with height reduce leaf water potential, a key metric of leaf water status. Despite the potential of trees to adjust key traits to minimize these effects, it is still hypothesized that the effects of gravity and path length reduce how effectively a tall tree can keep its canopy hydrated and resist the impact of drought events. However, to date very little data exist that can directly test whether the water transport systems of tall trees are negatively affected by tree height. RATIONALE: To study the effects of tree height on the plant water transport system, we studied trees from the tallest angiosperm family in the world, Dipterocarpaceae, which dominates the rain forests of Southeast Asia. We sampled 38 trees, from five species, ranging from 7.1 to 71 m in height. Measuring a suite of traits related to tree hydraulic function in addition to growth rates before, during, and after a strong drought, we sought to determine whether tall trees show systematic drops in water supply to their canopy, which could increase their vulnerability to drought and reduce their growth. RESULTS: We show that adjustments to woody vessel anatomy, including wider water-transporting vessels at the base of taller trees, compensated for the increased resistance to water flow associated with longer path lengths. Similarly, although leaf water potential did decline as the effects of gravity increased with height, so did the capacity of leaves to tolerate lower leaf water potentials, meaning that there were no negative impacts on leaf functioning. In addition, we found that the vulnerability of woody tissue to drought-induced embolism, a process that blocks the flow of water from root to leaves and makes them more prone to drought-induced damage, was not related to tree height. This result was further supported by no observed height-related declines in growth rate during a severe drought event. CONCLUSION: We demonstrate that height does not impair the water transport system of the world's tallest tropical tree species, increasing their hydraulic vulnerability or causing declining growth rates during drought as they grow taller. As these trees grow taller, their woody anatomy and leaf functioning adjust to fully compensate for the effects of path length and gravity. Our study suggests that changes in a tree's vulnerability to drought-induced embolism may be more related to canopy microclimate and shading than to height. Consequently, the widely adopted assumption that drought vulnerability increases with tree size needs to be more widely verified, as does the assumption that the negative effects of height on the plant water transport system can explain the elevated mortality in taller trees. Taller dipterocarp trees are not more hydraulically impaired than smaller ones.: Three-dimensional (3D) scans of dipterocarps span the studied height range. As trees grow taller, adjustments to their water transport system compensate for longer leaf-to-soil distances and stronger gravity effect on the xylem, resulting in no height-related impairment of function. A human and a three-story house provide scale. [3D scans by M. Disney and colleagues, University College London (UCL) Geography, UK] [ABSTRACT FROM AUTHOR]
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Database: Psychology and Behavioral Sciences Collection
Description
Abstract:Half of the aboveground biomass in forests is stored in a disproportionately small number of very tall trees. These giants are predicted to be more vulnerable to drought-induced damage because height impairs their hydraulic system. We evaluated whether the hydraulic system of world's tallest tropical tree species—Southeast Asian dipterocarps—are negatively affected by their height. The more negative xylem pressures caused by tree height were fully compensated for through adjustment of vessel anatomy and leaf hydraulic traits, and the trees suffered no height-related loss in growth during a severe drought. Therefore, height does not make the hydraulic systems of the world's tallest tropical tree species more vulnerable to drought, and the growth rates of these trees are not more negatively affected by drought than are their smaller counterparts. Editor's summary: Wood contains vessels that carry water from roots to distant leaves, acting like long straws. Vessels can collapse under low water pressure, which is harder to maintain at greater heights. Thus, water transport is thought to limit the maximum height of trees, and taller trees may be more vulnerable to drought. Whether these principles from temperate ecosystems extend to tropical trees, however, remains unknown. Bittencourt et al. investigated whether very tall tropical trees have traits that compensate for water stress by comparing trait measurements before, during, and after the El Niño drought period of 2023–2024 in five species of dipterocarps, the tallest known tropical tree family, in Borneo. The researchers found that hydraulic traits compensated for height-related declines in water potential, suggesting drought resilience of these massive trees. —Bianca Lopez INTRODUCTION: More than half of the carbon in forest ecosystems is stored in only 1% of the tallest trees. These tall trees are predicted to be more likely to suffer drought-induced damage under future drier climates. As a tree becomes taller, the distance from root to leaf increases, increasing the resistance to water flow. In addition, the greater effects of gravity with height reduce leaf water potential, a key metric of leaf water status. Despite the potential of trees to adjust key traits to minimize these effects, it is still hypothesized that the effects of gravity and path length reduce how effectively a tall tree can keep its canopy hydrated and resist the impact of drought events. However, to date very little data exist that can directly test whether the water transport systems of tall trees are negatively affected by tree height. RATIONALE: To study the effects of tree height on the plant water transport system, we studied trees from the tallest angiosperm family in the world, Dipterocarpaceae, which dominates the rain forests of Southeast Asia. We sampled 38 trees, from five species, ranging from 7.1 to 71 m in height. Measuring a suite of traits related to tree hydraulic function in addition to growth rates before, during, and after a strong drought, we sought to determine whether tall trees show systematic drops in water supply to their canopy, which could increase their vulnerability to drought and reduce their growth. RESULTS: We show that adjustments to woody vessel anatomy, including wider water-transporting vessels at the base of taller trees, compensated for the increased resistance to water flow associated with longer path lengths. Similarly, although leaf water potential did decline as the effects of gravity increased with height, so did the capacity of leaves to tolerate lower leaf water potentials, meaning that there were no negative impacts on leaf functioning. In addition, we found that the vulnerability of woody tissue to drought-induced embolism, a process that blocks the flow of water from root to leaves and makes them more prone to drought-induced damage, was not related to tree height. This result was further supported by no observed height-related declines in growth rate during a severe drought event. CONCLUSION: We demonstrate that height does not impair the water transport system of the world's tallest tropical tree species, increasing their hydraulic vulnerability or causing declining growth rates during drought as they grow taller. As these trees grow taller, their woody anatomy and leaf functioning adjust to fully compensate for the effects of path length and gravity. Our study suggests that changes in a tree's vulnerability to drought-induced embolism may be more related to canopy microclimate and shading than to height. Consequently, the widely adopted assumption that drought vulnerability increases with tree size needs to be more widely verified, as does the assumption that the negative effects of height on the plant water transport system can explain the elevated mortality in taller trees. Taller dipterocarp trees are not more hydraulically impaired than smaller ones.: Three-dimensional (3D) scans of dipterocarps span the studied height range. As trees grow taller, adjustments to their water transport system compensate for longer leaf-to-soil distances and stronger gravity effect on the xylem, resulting in no height-related impairment of function. A human and a three-story house provide scale. [3D scans by M. Disney and colleagues, University College London (UCL) Geography, UK] [ABSTRACT FROM AUTHOR]
ISSN:00368075
DOI:10.1126/science.aea9013